In this implementation of the Information Set Monte Carlo Tree Search, why can't the players see the cards of each other?
All the methods in the GameState class that is used to represent state, are stubs, and without these, the MCTS algorithm won't do anything at all. In particular, the DoMove method just changes who's turn it is, without actually taking any action.

Probably the reason the players can't see each other's cards is that this is not a completed implementation. Someone is either still working on this, or gave up half way.